Solid state membranes formed from oxygen ionically conductive materials are showing promise for use in commercial processes for separating oxygen from oxygen-containing streams. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification combined cycle (IGCC) plants. This technology encompasses two distinctly different membrane materials, solid electrolytes and mixed conductors. Membranes formed from mixed conductors are sometimes preferred over solid electrolytes in medium- and large-scale processes for separating oxygen from oxygen-containing gaseous mixtures because mixed conductors conduct both oxygen ions and electrons at elevated temperatures and can be operated without external circuitry such as electrodes, interconnects and power-supplies. In contrast, solid electrolytes conduct only oxygen ions and require such external circuitry to be operative.
Membranes formed from solid electrolytes and mixed conducting oxides are oxygen selective and can transport oxygen ions through dynamically formed oxygen anion vacancies in the solid lattice when such membranes are subjected to temperatures typically above about 500.degree. C. Examples of solid electrolytes include yttria-stabilized zirconia (YSZ) and bismuth oxide. Examples of mixed conductors include titania-doped YSZ, praseodymia-modified YSZ, and, more importantly, various mixed metal oxides some of which possess the perovskite structure.
Membranes formed from mixed conducting oxides which are operated at elevated temperatures can be used to selectively separate oxygen from an oxygen-containing gaseous mixture when a difference in oxygen partial pressures exists on opposite sides of the membrane. Oxygen transport occurs as molecular oxygen is dissociated into oxygen ions which ions migrate to the low pressure side of the membrane where the ions recombine to form oxygen molecules while electrons migrate in a direction opposite the oxygen ions to conserve charge. The rate at which oxygen permeates through the membrane is mainly controlled by two factors, the diffusion rate within the membrane and the kinetic rate of interfacial oxygen exchange; i.e., the rate at which oxygen molecules in the feed gas are converted to mobile oxygen ions at the surface of the feed side of the membrane and back again to oxygen molecules on the permeate side of the membrane.
Membranes formed from mixed conducting oxides offer substantially superior oxygen selectivity than polymeric membranes. However, the value of such improved selectivity must be weighed against the higher costs associated with building and operating plants employing membranes formed from mixed conducting oxides because such plants require heat exchangers, high temperature seals and other costly equipment. Typical prior art membranes formed from mixed conducting oxides do not exhibit sufficient oxygen permeance to justify their use in commercial gas separation applications.
Japanese patent application 61-3-4169 discloses an oxygen permeation apparatus which utilizes a membrane formed from a mixed sintered body consisting of Sr.sub.(1+x)/2 La.sub.(1-x)/2 Co.sub.1-x Me.sub.x O.sub.3-d and SrMe'O.sub.3 where Me=Fe, Mn, Cr or Va, 0&lt;=x&lt;=1 and Me'=Ti, Zr and Hf. The examples state that modest improvements in oxygen anion conductivity can be achieved by impregnating the entire surfaces of such membranes by immersing the sintered membrane bodies into solutions of silver-, palladium- or platinum-containing compounds.
Solid State Ionics 37, 253-259 (1990) further describes the membranes presented in Japanese patent application 61-3-4169 wherein palladium metal is added to the mixture of metallic oxides prior to sintering the mixture of metallic oxides to form a palladium-containing multicomponent metallic oxide. Sintered samples containing palladium showed a higher "oxygen anion conductivity" than samples which did not contain palladium.
U.S. Pat. No. 4,791,079 teaches novel mixed ion- and electron-conducting catalytic ceramic membranes consisting of a first layer of impervious mixed ion- and electron-conducting ceramic material and a second layer which is a porous catalyst-containing ion-conducting ceramic material. A preferred composition for the second ion-conducting layer is zirconia stabilized with 8 to 15 mole % calcia, yttria, scandia, magnesia and/or mixtures thereof. The reference neither teaches or suggests forming the porous layer from a mixed conducting ceramic material, namely, a material which conducts both electrons and oxygen ions at elevated temperature. The membranes are suitable for use in hydrocarbon oxidation and dehydrogenation processes.
Researchers are continuing their search for thin, ceramic membranes which exhibit superior oxygen flux and sufficient mechanical strength and properties to enable their use in commercial processes.